A strain of the fungus, Phytophthora infestans, causes late blight in tomatoes. As the common name implies, Phytophthora is prevalent on tomato hosts in late summer, after the plants have bloomed. Late blight is more common in north central and northeastern states, but is observed in the Midwest when the humidity is high and temperatures are around 68 degrees F late in the growing season. Watch for the disease when cool, moist nights are followed by warm, humid days.

Symptoms and Diagnosis

One of the first symptoms of late blight is watersoaked patches on older leaves. Late blight attacks the older leaves first, then spreads to the fruit. Green-black watery patches develop on the upper surfaces of older leaves. These patches will enlarge quickly, and in moist weather, a downy growth may develop on the underside of the leaf. On the fruit, you will see rough, firm, dark-colored spots.

Life Cycle

The fungus overwinters on tomato and potato plant debris, including potato tubers. The fungus can produce spores over a broad range of temperatures although spores are most infective at temperatures of 68 degrees F with high humidity. The spores are transmitted by water or are wind blown and may be introduced from diseased plants in nearby gardens. The fungus invades the plant through leaf stomata. Infection rarely occurs when temperatures are higher than 80 degrees F and humidity is less than 90%.

Integrated Pest Management Strategies

1. Plant resistant varieties. Several tomato varieties are designated as resistant to late blight.

2. Avoid overhead watering. Avoid working around plants when they are wet. Both of these practices can spread the fungal spores from plant to plant.

3. Clean up all garden debris in the fall. Remove and destroy any affected plants as soon as they are observed.

4. Practice crop rotation. Do not plant tomatoes, potatoes, and celery in succession. All of these crops are susceptible.

5. Use a copper-based fungicide. Apply when symptoms are first identified. Reapply at 7–10 day intervals.